In the past, a control system of an internal combustion engine which provides an air-fuel ratio sensor in an exhaust passage of the internal combustion engine and controls the amount of fuel fed to the internal combustion engine based on the output of this air-fuel ratio sensor has been widely known. As such a control system, one which provides an air-fuel ratio sensor at an upstream side of the exhaust purification catalyst provided in the exhaust passage of the engine and provides an oxygen sensor at the downstream side has been known (for example, PLTs 1 to 4 etc.)
For example, in the system described in PLT 1, feedback control is performed based on the output of the upstream side air-fuel ratio sensor so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes a stoichiometric air-fuel ratio. In addition, deviation can occur in the output of the upstream side air-fuel ratio sensor, so the output of the upstream side air-fuel ratio sensor is corrected based on the output of the downstream side oxygen sensor. Furthermore, this system obtains the amount of correction of the output of the upstream side air-fuel ratio sensor based on the output of the downstream side oxygen sensor at certain time intervals at a certain ratio as a learning value to thereby update the learning value and uses this learning value for correction of the output of the upstream side air-fuel ratio sensor.
In addition, the system described in PLT 1 shortens the time interval of obtaining the learning value and increases the ratio of obtaining the learning value when a mechanical compression ratio set by a variable compression ratio mechanism is high so as to increase the speed of obtaining the learning value. Due to this, according to the system described in PLT 1, even when the mechanical compression ratio is high and therefore the ratio of the unburned HC contained in the exhaust gas is high, it is considered possible to quickly determine the learning value.